Butter cream is one of the few basic creams which have been used for centuries in cake filling and masking. Basically, it is a whipped butter with the addition of sweet components like icing sugar, fondant etc.
Egg is not used in every butter cream but it has the duty of making butter cream more lighter and tastier. Lightness also come from the amount of whipping time which puts more air in to it. Having said that, smooth butter cream should not have much air, especially for masking.
You can flavour butter cream easily with coffee, chocolate, nougat, lemon paste and so on. Pretty much any kind of flavouring which has a very low water content is ok. It is a little more tricky to use fresh fruit. Fresh fruits must be cooked first to remove the water content as much as possible. This will take away acidity from the fruit as well so to regain that fresh acidic bite, a few pinches of citric acid powder can be added.
There are four different main methods of making butter cream. Each with their own national identities.
ITALIAN or SWISS
Butter whipped with hot syrup meringue.
I wouldn’t call my recipe Italian because I do not boil my sugar syrup to add in to egg white but this method is probably the closes to mine.
Butter whipped with custard cream.
Butter whipped with sabayon.
AMERICAN or ENGLISH
No eggs used. Icing sugar or fondant, maybe some milk or cream added.